Process for the production of sulfur trioxide



A. BELCHETZ Sept. .'14, 1948.

PROCESS FOR THE PRODUCTION OF SULFUR TRIOXIDE Filed Aug. 5. 1945 Patented sept. 14,1948 t ,1 2,449,196

UNITED: ASTATES rarsN-rr OFFICE PROCESS THE PRODUCTION OF SULFUR TRIOXIDEV Arnold Belchetz, Larchmont, NL YL, assigner tot" Staufer Chemical Company, alcorporatonof California Application August 3, 1945, Serial No. 608,742'

` iolaims; t (C1. 25a-174.) 1 2f Thisinventionrelates tothe production of sul# i l the rst stage of conversion, while the last-fstage` phurV trioxide in av concentration suitable for is usuallyseveralfeetindepth.- rrianuiacturev `or sulphuric acid or for other in It' iscustomary to"-introducethey gas atf tem dustriallutilization. peratures of' aboutllO4 Eto 850 Ff into xedi `5 bed converters, to ensure that therealctiontwill` THEv PRIOR ART proceed` at` a,- sufficiently` great,T rate` to ireache equilibrium for the reaction 2SO2+}O2 2SO`3, The-use of Very shallow catalyst bedsinthe-rst and second conversion` stages' results` inan-e'X- tremelyV short tixne'ofcontactjofgthe'gas withtlfler catalyst, usually only a vfraction. of al second, so thateonditions conducive to -a-h'igh rateoffre#- In the manufacture' ofsulphur trioxide for l subsequent `conversion into sulphuric acidifrom sulphur, it is usual' first to burn sulphur with air to form sulphurdioxide. Subsequently, the sul-` phur dioxide' is mixed With'additionalzairand is then passed'over a suitable catalyst to form sulphur trioxide from the air-sulphurdioxidemixn ture in accordance with thefollowing reaction: Converslon m` these'eonver'elen"etagee Ifthe` inlet temperatureof the gasisreduced; thetemf-f 2SO2+02 2S03` perature reached in-theconverter willfalll andE the rate of'reaction^wi1l"drop. llfiecatalystlbed-v actioninust be maintained to attaimthe desired The ee'teilyet mees represents e eonsdemble Per' will' gradually become colder as-the temperature* tion of the capital investnfientI in asulphuric acid of; the, inlet, gas-is decreased, and a), point `will;` Pll'ltifl the mass must be manufactured? Cenel- 20 eventuallyy bereaehedfwhen the-conversion willf tioned for use (SeeKOet-Z Patent 1,941,557') and t becomeuneconomicalorwherethe'reactionmay placed in the` converters, usually byV hand." The Cease completely l material or materials comprising the catalyst are'` The equilibrium `fm.'tHe-.1realm-fion,

usually disposed on particles of a carrier mate` rieL. such` as silica gel, diatomaceousl earth, asv` 25 250244326250? l bestes bl'e, ,magnesium Sulphate, etc- Slcethe is `favoredvbyrelativelyflow temperatures and a CatalySt' iS emDlOYGd in 2J Xed 01' Stal/l0 bed; `the l temperature of about 800; isf desirablegif: `a t bed must be sufcientlylshallew end of sufierltly-y high -conversionltorsogi isf desired;` Asltheem-:l\` t t great diameter; andthe carrier particles must be perature of` the A,exit gases` is increasedathexreew of al size anda shape so as to produce the least actionisreversed;,setthatfin xedrbed units-it=is possible pressure drop in the gasstreamfpassing t customarytocarry out the conversiondntwoor:A through the catalyst chamber, to reduce :power-` more.stages,wth intercoolingtbetweenstagesytoy` costs, blower costs, etc. attainthe desired nalconversion.

The reactionI of SO2` and oxygen to form suli The following. :areA tyiiicalz-temperature` condi# phur trioxidey is quite exothermic. The most tions for atmodernB-stageixedebed conversionfavorable eoulilibrium` for conversion of "SO2 to unitl on an inletfgas; containingf- 91156)A auchV S03y is realizedatabout 800 F. although' the rate l wherein the iinal` overall4 conversion ofSOza to '1 t of reaction is greater at higher temperatures." SOawas 98.5%: t If one attempts the conversion inI a single pass throughthe catalyst in a fixed bed", the naltem- 11 Inletonly Outlet pement Conversion@ i perature reached will depend upon the degree of K Temperature Temperature OLS l soa dilution ofthe S02 and oXygenwith-,inert gases,` T tst l F y such as nitrogenf or carbon.dioXde;ahd' upon "irS age--l 755-760; 1,100-1115@ 0m 120,64-, themas or` threshold-temperature ofthe gasf fslids; g lgltg iet`t3'-9ttia mixture. t 45 w The usuelpreetee le te provide the Catalyst Theabove iiguresiillustrate` thatiwitlfi;norma-lt messi et' lease two' separetebeds'iend to e001* concentration oirSOziinfthe inletgas, ,tempera-p the' gas stl-"eem between ee'eh bedj T0 ensure tures 'reached iniiixed bedi'converterslareftoofhighrr reaching equilibrium, S0 as toebtelrr maximum mpermitlof reachingigood:conversions ory sog, conversion `in each Dass,- tlle Sel/'eral Catalyst 50 toSOminone'stage.. `'Ihe.temperaturerise:coulda, y masses are made of increasing volume-anddepth be reduced)byrdilutingthe SO2 wthtain or flue;` tepemt 0f longer ContactA periods as the SO2 gas, but the Volumes-ofwgasestobehandledr would .f andOz concentrationsdecreaseandthe `S03 corr--A beto@great'forfeoonomicaloperation.-

l centration increases inl thefgasstream. A cata- It-Will-fbe obvious *that atthe xmostlthe tempera--` lystmass depth of afew inches ausuallysurlees in ture-'across austatici` catalystmassacanf only'bear averaged. Consequently, the usual practice is to carry out the conversion of SO2 to S03 in stages with intercooling, taking most of the conversion in the first, and second stages, Where higher average temperatures can be tolerated, and to use the final or third stage for clean-up purposes, at a lower average temperature, to attain a high overall degree of conversion.

With the passage of time, dust and other foreign solids collect in the static catalyst beds, gradually increasing the pressure drop across the bed so that at periodic intervals the catalyst chambers are taken off stream, the catalyst removed and replaced, thus resulting in process interruption. The gas stream passed to the catalyst mass must therefore be carefully freed of solids; even with this practise, the resistance to flow of gas increases rapidly with use. To facilitate temperature regulation and to permit the catalyst mass to be freed of foreign solids, various complicated and intricate apparatus constructions have been suggested, none of which has -provided a completely satisfactory solution to keither problem.

THE USE F OTHER'SULPHUR SOURCE MATERIALS So farI have mentioned only the production of sulphur dioxide-from sulphur. It is also usual in the art to produce sulphur dioxide from hydrogensulphide, hydrocarbons containing sulphur such as disulphides and thioethers, strong used sulphuricacids such as alkylation sulphuric acid sludge, alcohol acid sludge and the like, and acids or sludges having a smaller acid content such as those which result from the refining of petroleum distillates; all of these sludges result from the industrial use of the acid in manufacture of various materials by known methods. To eiect decomposition, the combustible materials are burned While the so-called spent acids are heated under such conditions that the acid is decomposed to form volatile hydrocarbons, hydrocarbon gases, water, carbon dioxide and sulphur dioxide.

If any considerable quantity of hydrocarbons is present, the removal of these from the sulphur dioxide-water-carbon dioxide stream becomes a problem and the provision of hydrocarbon condensers and hydrocarbon combustion chambers ahead of the converters is usual. If the gases are subjected' to combustion, the sulphur dioxide stream passing to the converters is further diluted by the carbon dioxide formed in the combustion and by nitrogen introduced with the oxygenff'orfcombustion. This increases the volume of the catalyst mass required for if a conversion of the order of 94% and better is to be secured, a necessity in any commercial operation, adequate contact time must be provided between the catalyst mass and the gas stream. Obviously, the leaner this stream is in SO2, the greater will be the catalyst mass required for a given degree of conversion to produce a fixed or equal quantity of S03.

A further disadvantage, inherent in sludge decomposition plants or in plants in which hydrogen `sulphide or hydrocarbons containing sulphur are burned as a source of SO2, is that water is formed'nponl combustion of the hydrogen present in addition to the Water originally present in the air. It is customary to scrub the gases formed upon sludge acid decomposition or combustion with large volumes of coldwater, to condense out the bulk of the Water. The gases are then thoroughly dried after cooling, by scrub` bing with acid to remove the last traces of moisture. The dry gas stream is then permitted to enter the converters for conversion of SO2 tol S03. The product gases leaving the converters. can now be cooled and the S03 absorbed in 98.5%- H2SO4 without danger of condensing water containing S03 or SO2 which would be extremely corrosive. The complete drying of the gas before entering the converters is also regarded as necessary to avoid the formation of troublesome acid mists.

THE PRESENT PRocEss In accordance with this invention, all of the foregoing diiiculties are obviated While a simpler, more direct and less involved process is provided. Broadly, the invention contemplates combustion of one or more sulphur source materials in the presence of catalyst for the oxidation of SO2 toV S03, the lcatalyst being present as a suspension in the air or oxygen containing gas required for oxidation ofthe sulphur to SO2 and SO2 to S03. The process of the present invention contemplates the mixing of the catalyst and air or oxygen containing stream with the sulphur (air is used herein and in the claims as referring to a gas containing sufcient oxygen to be useful in process; it therefore includes air, pure oxygen, air enriched with oxygen or a gas containing suicient oxygen in the presence of one or more other components which do not interfere with the oxidation). The catalyst is preferably utilized to introduce heat into the sulphur source material to assist in vaporization thereof and to promote primary combustion to form sulphur dioxide. The same catalyst then serves to promote the secondary oxidation of the SO2 to S03, in the presence or absence of carbon dioxide or Water, thus simplifying the process and the apparatus employed. One is thus able to provide a single compact plant capable of operating upon any one of various sulphur source materials, or combinations of these, Vto produce sulphur trioxide in a'concentration suitable for production of sulphuric acid. Temperature control in the conversion zone is effected by recycle of a cooled stream of catalyst suspended in air or oxygen .containing gas. Suiicient heat is stored in the cooled catalyst stream to eifect instantaneous vaporization or decomposition of the sulphur source material, which is injected into the stream of cooled catalyst suspended in air or oxygen containing gas. Primary combustion of the sulphur to SO2, carbon t0 CO2 and hydrogen to water, will be effected in an extremely short period of time, followed by the slower secondary oxidation of SO2 to S03. The total heat removed from the mixture of hot recycle catalyst and air or oxygen containing gas must equal the total heat liberated in excess of that quantity required to bring the products of combustion and oxidation to the conversion temperature maintained in the conversion zone. The excess heat evolved in conversion of the sulphur to sulphur dioxide, of the sulphur dioxide to sulphur trioxide and from the combustion of the hydrogen and carbon present, can be utilized for production of steam required in the process for driving pumps, compressors, etc. Ordinarily, the heat available from combustion of any hydrogen and carbon present and upon formation of sulphur trioxide, while suiiicient to require cooling, is not put to any practical use. By having these available in one reaction zone, the process of this invention enables a maximum of heat recovery either as such or. for]V utilization in the vaporization and, com'- bustion of additional sulphur source material.

Therquantity of' catalyst circulated is dictated byf theV heat liberated and the temperature con,-y trol required in the conversion. The catalyst is usually disposed on a suitable carrier such as diatomaceous earth, kaolin, asbestos' libre, and the like, the useful carriers being well` known in the Y art. Ii necessary a solid4 diluent, consisting of some inert material,` can be added to the catalyst, to serve` as a heat control and a heat abstraction medium, diluting the catalyst and taking up the heat of, comb-ustion and of conversion.

While it is known that use of an elevated pressure provides a more favorable equilibrium for conversion` of SO2 to S03, so far as I am aware no one has as yet been able to provide a practical process capable of operation at pressures.

up to 300 pounds gauge. The process oi this invention is particularly suited to operation at a pressure elevated above atmospheric although such operation is not a necessity.

It is in general the broad object of the present invention to provide a novel process for the manufacture of sulphur trioxide from suitable sulphur source materials.

Another object of the present invention is to provide a novel process in which sulphur trioxide is manufactured from a sulphur source material and air in the presence of a finely divided catalyst present in gaseous suspension and utilized to con trol combustion and conversion temperatures.

A further object of this invention is to provide afluid catalyst process for manufacture of sui'- phuric acid utilizing a suitable catalyst to control combustion and conversion temperatures.

i A still further object of the invention is to provide a fluid catalyst process utlizing a suitable catalystfor manufacture of sulphur trioXide from a suitable sulphur source material and which is carried on at an elevated pressure.

The invention includes other objects and fea-- tures of advantage, some of which, together with the foregoing, will appear hereinafter, wherein the` present preferred manner of practising the invention is disclosed. The single gure in the drawing accompanying and forming a part hereoi is a diagrammatic apparatus representation and a ow sheet useful in practising the invention.

APPARATUS Referring to the flow sheet, the apparatus there shown includes a molten sulphur feed pipe l, delivering sulphur in a liquid state to sulphur feed tank 8. The molten sulphur is withdrawn from this tank throught pipe 9 and is discharged by pump li through pipe IZ into another pipe i4 which extends through the bottom of the converter il and discharges into the apex of an open cone iii. A standpipe I8 is provided'at the bottom of chamber ll extending downwardly from the chamber Il to another pipe I9 which leads to catalyst cooler 2 I. Catalyst in an aerated con.n dition flows from the bottom` of the converter H through the ow control valve 'i6 into the line l5. A side outlet pipe 23- is provided on pipe i8 to permit Withdrawal of catalyst as desired while several pipes 2G permit air to be introduced into the standpipe i8 when required to maintain the' catalyst in the standpipe in an aerated condition and to control the density of the catalyst flowing down the standpipe I8.

A suitable heat exchange huid such as water is introduced when the apparatus is in'operation into the catalyst cooler 2|Y through inlet pipe l22` 6i fromsteam1 separatori 214.; Water is injected into thesteam separator` from inlet pipe 26 by pump 2lthrougllv. line 28 Heated water and steam are removedfromlthe catalystccool'er 2l through pipe 29f and steam is removed from the steam sepafroml the presently described air dryer l2 through line 'ML For theA initial catalyst addition or toA replace catalyst lost or deactivated in the, operation, the catalyst is charged into a catalyst hopper All and discharged by means of a screw pump l2rinto line 43;. air being supplied by pipeyll` from' pipe 33 to convey the catalyst through pipeA :23a-nd into pipe IS The great bulk of the catalyst gases before passing out of the converter, a level of catalyst in a` fluidized condition `being maintained in converter IT. When operating on sulphur, the gaseous products and catalyst iines not separated in `converter l1; are removed from the top of. converter l1 by4 pipe 5i directly: into a sulphur trioxide cooler-52 and' by-pass 8l is` not employed'. The gasa, stream from cooler 52 is t' passed through pipe; 53rint`o-a suitable. dust separating unit which have Aindicated at 54 as' a Cottrell precipitator; one of the electro-static type, to remove what catalyst iines are present. As previously mentioned, the bulk of the catalyst remains in the converter 4il and iiows through the standpipe ifor recirculation. The precipitator delivers the separated iines to a pipe 55 and into a screw-fv pump 511 whence they are picked upy by air delivered from pipe 33 and conveyed through pipe 34finto pipe I9. The gaseous products, freed fofcatalyst nes, pass through pipe 6l into the sulphur. triorridev absorber E2, into which water'or dilute acid is admitted from line 63.

Catalyst can bei Withdrawn through pipe 23 and fresh catalyst-added' as `desired from hopper 4I.. The catalyst need" not' be especially formed intoy special shapes or'sizes, but should be fine enough to'permit of itsfbeing fluidized. The cata-` lyst should' preferably beline` enough to pass through a 20D-mesh screen, but should contain only a small percentageofmaterial of less than 10 micron particle size so asto avoid excessive carry-over offine's=,fromI vessel` 17. The catalyst can be conditionedintheconverter and need not be iirst externally heatedf-in-anSOz atmosphere, as is usually required in preparing ixed bed catalyst. Theprocess also enables the catalyst to be employed at a pressureelevated above atmospheric and favorable tothel formation of S034 at` the temperature of4 operation. Operation at elevated pressuresK is not Afeasible in present known processes. Although'pressuresY as high as 300 poundsy gauge are practical, 1 prefer to use pressures below 100 pounds gauge and preferably between l pound and poundsigauge.

"Acid is withdrawn through line iii!v at thel base of` the absorber 62 and `is passed through acid cooler G6. From the cooler, a` portion of acidris injected into` converter IT through line i4 separates from theother portion is circulated by pump 61throughpipe 1I to the air drying tower 12. Acid is drawn 01T at the base of the tower 12 through pipe 69 into cooler 15 and is then circulated by pump 55 through line 68 to the absorber G2. Pipe 13 admits air from the air compressor 38 to the air dryer 12. The air issues from the dryer into line 14 connected to pipe 33. Resi-dual gas is released from absorber 62 through pipe 10 under the control of pressure regulating valve 80.

OPERATION oN SULPHR ture at which moisture can condense. When this has been attained, the catalyst, in 'lnely divided form, is gradually delivered by screw pump 42 from the catalyst hopper 4I into the air stream 44 and the suspended catalyst mixed with preheated air from line 31 andv conveyed through the catalyst coolerZI into the converter I1. Addition of catalyst is continued until the desired level has, vbeen built up in the converter I1. The catalyst is 'circulated with theV preheated air stream until a'temperature of the order of 800 F. has been reached in the entire system. During the whole of this preheating period, air leaving the Cottrell precipitator is discharged directly to the atmosphere instead of passing through the sulphur trioxide absorber. As an alternative, the sulphur trioxide absorber can be emptied of acid and the air can be allowed to pass through the absorber to the atmosphere.

When the system has been brought to proper temperature, the air heater 36 is by-passed, sulphur injection is started and water circulation through the catalyst cooler is commenced. Circulation of catalyst and water through the cooler 2| is regulated to remove the excess heat of reactionand to control the temperature in the converter to secure the greatest S03 content in the gases passing through line 5I. The air from the compressor 38 is passed through the air dryer 12. The gases leaving the converter pass through the S03 cooler v52 wherein their temperature is reduced from '750 F- to 300 F. The gas stream passes through the Cottrell precipitator and the clean reaction products enter the SOa absorber 62 where the S03 is absorbed in 98% H2504. The residual unabsorbed gases'are released to the atmosphere from the absorber at a pressure of about 40 pounds gauge'through pipe 10. Acid coolers 66 and 15 remove the heat of absorption of SOaand moisture in the acid streams.

Example I In one installation, operating with liquid sulphur injection and a catalyst composed of a mixture of finely divided diatomaceous earth impregnated with vanadium pentoxide and potassium sulphate, liquid sulphur was injected by pump I I into pipe I4 at 250 Rand at a rate of 3,085 pounds' per hour. The Idry air supplied from tower 12 was suflicientA to Vsupply a 10% excess of oxygen'over that required for conversion of the sulphur to SO2 and the SO2 to S03. The heat liberated by combustion ofthe sulphur to SO2 would have been suilicient to raise the temperature of the gas to above 2000 F. if no catalyst were present to absorb the liberated heat. However, the catalyst was circulated at a rate of 500,000 pounds per hour to maintain a temperature of 750 F. in the converter, a 98% conversion of SO2 to S03 being obtained. A catalyst concentration of 3.42 pounds per cubic foot of gas entering the catalyst cooler was maintained, the temperature of the mixture entering the cooler being 716 F. at 48.4 pounds gauge pressure. The outlet temperature from the catalyst cooler Was 610 F. I have found this catalyst loading to be suitable for an operation of this type. The catalyst cooler produced 12,400 pounds per hour of steam at pounds gauge pressure, the cooling duty being 12,720,000 B. t. u. per hour. The composition of the gas passing through pipe 5I was as follows:

Mol per cent Oxygen 2.0 Nitrogen 84.3 Sulphur dioxide 0.3 Sulphur trioxide 13.4

111 tons of 100% sulphuric acid were produced per 24-hour day in this operation.

The apparatus is quite simple as compared with the usual sulphuric acid plant. For example, the converter, in the operation described, consisted of a cylindrical, unlined steel vessel 6 feet in internal diameter and 30 feet high, while standpipe I8 was 12 inches in diameter and 30 feet long; the catalyst cooler was made of l-inch tubes, 16 feet in length. Except for the S03 absorber, which was lined with acid brick in the usual manner, and the air heater, which was lined with fire brick, the entire plant was made of ordinary unlined mild steel.

The uidized catalyst bed extended approximately 20 feet above the bottom of the chamber, the supercial gas velocity in the chamber being 1.5 feet per second and the catalyst density approximately 15 pounds per cubic foot. The gas contact time was approximately 13 seconds, a very substantial increase in Contact time over that normally provided in the usual contact plant, ensuring attainment of equilibrium and substantially complete conversion. A gas velocity of about 30 feet per second is maintained in the piping through which suspended solids are conveyed. A solids loading of about 3.4 pounds per cubic foot is maintained in the air stream in pipe I9, the catalyst-air stream flowing at the rate of 30 feet per second. A catalyst concentration of about 25 pounds per cubic foot is maintained in standpipe I8. The length of standpipe I8 is such that with a catalyst density of about 25 pounds per cubic foot in the standpipe, pressure built up by the downward flowing catalyst will be sufficient .to maintain a pressure diierential of one to two pounds across catalyst iiow control valve 16, to prevent air backing up from line I9 into the standpipe IB.

CATALYST CIRCULATION In order to keep the temperature of the gas down to a point at which high conversion of SO2 to S03 Will occur, it is necessary to circulate the cooled catalyst in sufficient quantity to take up the heat liberated in the combustion of the sulphur to SO2 and in the oxidation of the SO2 to S03. The quantity of catalyst which must be circulated will depend upon the temperature which it is desired to hold in the converter and theextent or temperature to which the circulated catalyst is cooled. Thus, supposing vit is desired to hold a temperature of 750 F. in the converter, then if the circulated catalyst is cooled to 600 F., twice as much catalyst must be circulated as would be the case if the catalyst `were cooled to 450 F. Another extremely important feature is the temperature which it is desired to hold in the converter. Thus, if the converter temperature is held at 900 F., the reaction products will remove a greater proportion of heat :than would be the case if the converter temperature were held at 750 F. The amount of heat which is removed by the reaction products will also depend upon the degree of dilution of the S03 with nitrogen and oxygen. Thus, a more dilute gas will remove a greater `proportion of the liberated heat than will a more concentrated gas. For this reason, it is difficult to draw an empirical equation governing all situationsand all operations, since there are so many variables. For this reason, I wish to point out the range of reaction temperatu-res, the range of proportions of liberated heat removed by the reaction products :and the range of weight ratios of catalyst circulated to sulphur charged. The ranges applicable to sulphur will become apparent from Jche following:

Converter Tempera' 750r. 900 r. 11oc FJ Percent SO2 in gas immediately after combustion of sulp iur Percent Heat of Reaction removed by Reaction Products..

Percent Heat of Reaction removed by Cooling of Catalyst.

l2. 75 8. 0 l2. 75 8. O 12.V 75

Temperature to which circulated Catalyst is cooled Pounds of Catalyst circulated per, pound of sulphur injected i 1 l100 F. is too high a converter temperature for good efficiency, unlress ta second or clean-up converter is used in line 5l ahead oi absor er i2.

2 Cooling of the catalyst to 300 F. is considered impractical.

One can app-ly a formula to the situation and the following may be helpful in determining the catalyst to sulphur feed ratio:

Let

H=heat liberated in oxidizing 1 pound of sulphur to S03 (as B. t. u.).

i=heat removed by reaction products (as 1B. t. u.

per pound sulphur oxidized to S03).

Ti=iconverter temperature (in FL).

T2=temperature to which recirculated catalyst is cooled F.)

S=specflc heat of catalyst (about 0.23).

Then neglecting heat lost byradiation tothe atmosphere, which will be small for` aninsulated system,

R=pounds catalyst circulatedlper pound of sulphur oxidized to S03.

R 1 To S Sufficient catalyst must be circulated toensure that adequate heat abstraction can bemade from the catalyst `mass to the end'tl'iatv the temperature in the converter does not exceed that at which the maximum conversion is lattained for the operating pressure andthe `SO2 'and O2 concentrations. Since the weight of catalyst circulated can be readily controlled relative to the quantity of sulphur fed, the catalyst quantity can be adjusted with preciseness until themaximum conversion eiiiciency is secured.

lIn `the case of sulphur, Vthe catalyst-sulphur yfeed ratio should be adjusted `to maintain the Example I I A spent alcohol sulphuric acid was "injected through line 30, thegacid containing 78% "H2504, 0.35% hydrocarbons (as CH2) and 21.65% water, 40.2 pounds of sulphur were also injected through line 30 into line vIll foreach 100 pounds of spent alcohol acid. The acid canbe introduced directly into the reactor, if desired. VThe,product from the reactor contained equimolar lproportions of sulphur trioxide and water to form sulphur` acid. Operating upon this basis, a total of 198.3 tons per day of 100% sulphuric acid were produced of which 78 tons `per dayor 39.3% `were provided by the spent alcohol acid and 120.3 tons or 60.7% were produced `from the sulphur. When the spent alcohol acid was injected .at 100F. and the converter operated at .750 F., `the excess heat removed by cooling the recirculated catalyst amounted to 4,570,000 B. t..u. per hour.

It should loe-emphasized in this example `that thespent alcohol acid does not containsuflicient combustible `material .to provide the heat necessary to vaporizethe spent alcohol acid andsuper heat 4tlrievapors to 750 F. Since at 750 F. the sulphuric acid `will be almost completely dissociated to SOB and H2O, the additional heat of Jdssociation and `the heat of `vaporization .and superheat must be provided by combustonand oxidation of thejsulphur injected. p

When operating with a mixedsludgesulphur feed, it is essential that the catalyst fines be removed before the reaction gases are cooled to a point where sulphuric acid condenses. For example` in the :operationwith themixed alcohol acid-sulphur feed, if the converter is `held :atrial pound gauge outlet pressure,V `the partial pressure of S03 and H2O is 19.5,.pounds absoluteor 4.8 pounds gauge. At this partialpressure. sulphuric acid begins `to condense lai; 650"F. Ifltlie foonverteris operated at 750 it is obviously necessary to remove the catalyst fines `from `thereac tion i' products `before any substantial cooling of these gases is effected. Tcefectthis, by-passfl is-provided around theSOs cooler `and thereaction productsare `passed directly `to the Cottrell precipitator for removal of' the ycatalyst lines. When this isachieved, the gasisthenpassed-into the sulphur trioxide absorberwhiclr'in this case, functions primarily as a sulphuricacid condenser.

'While-in the foregoing I havermentioned'that `provide equimolarrproportions of S03 and H2O,

so, that the resulting product, upon condensation, wasw100%n acid, this is not necessary and one can use other proportions. However, if the proportion, of the sludge be increased, then the water content of the reaction products will be increased and the acid may require fortification with S03 to produce 98.5% acid or stronger acid. Conversely if it is desired to produce oleum, the proportion of alcohol sludge acid to sulphur injected should be reduced.

HYnRocARBoN CoMBUsrroN t TheV combustion of the hydrocarbons does not prsent' any particular problem if a vanadium oxidationcatalyst is utilized, for such a catalyst promotes oxidation of hydrocarbons to CO2 and water. Thus, while in other processes extremely high temperatures (1000 F.-1600 F.) are required to decompose a sludge, Itotal sludge decomposition and combustion can be effected in the An alkylation acid sludge, containing 87% I-I2S04, 8% hydrocarbons (as CH2) and 5% water, was introduced continuously with the sulphur, the rates being U tons per day of the sludge and 27.2 tons per day of sulphur. From this operation, 170.3 tons per day of 100% II2S04 were recovered, while 11,850,000 B. t. u. per hour excess heat was removed by cooling the recirculated catalyst. @Injection of the sulphur is not necessary for the decomposition of alkylation sludge acid, since the heat liberated in combustion of the hydrocarbons will be suilcient to vaporize the sulphuric acid and water present, superheat the vapors to 750 F. or 900 F., if desired, and dissociate the sulphuric acid completely to S03 and H2O. If sulphur is not injected with the sludge acid, the sulphuric acid will be recovered in a strength below 98.5% and will require fortiiication with S03 before being reused for alkylation purposes.

WEAK Acro SLUDGE As the hydrocarbon content of the sludge increases, the acid strength generally decreases. The volume of carbon dioxide, water and Vnitrogen formed on decomposition and burning of the hydrocarbons increases and the handling of a large volume of gas becomes necessary. Since the exothermic heat liberated by the combustion of the hydrocarbons becomes greater as the proportion of hydrocarbons increases, a greater circulation of catalyst is required with increased cooling for temperature control. The S02 content of the gases.` is small compared with gases from combustion of other sulphur source materials. It is accordingly preferable to burn sulphur with a sludge of this type and thus insure recovery of an acid of useful strength, since the combustion of hydrogen in the sludge vproduces a large 12 proportion of water Vapor. To illustrate operation on a sludge of this character, ther following example is given. y

Example IV 46.0 tons per day of a pressure distillate sludge (42.3% H2804, 50.5% hydrocarbons as CH2 and '7.2% water) were injected together with 58.8 tons per day of sulphur. The converter was operated at 44.1 pounds gauge outlet pressure and at a temperature of 750 F. The sludge-sulphur quantitieswere chosen to provide equimolar quantities of S03 and water to form 100% acid. 200 tons of 100% acid were recoveredper day and 54,200,000 B. t. u. per hour were removed in the catalyst cooler by cooling the catalyst to 60 F.

SUMMARY In the following table are set forth certain pertinent data and apparatus details. These are given on a common basis, production of 200 tons of 100% acid per day:

Case 2, Case 3, Case 4 Case l, Sulphur- Sulphursul bu'r charge sllmr Aspetl sielnnl. PDD- y co o y a yion Acid Acid Sludge Composition of Spent Acid:

Per cent CH2 0. 53 8.0 50. 5

Per cent H20 21. 65 5.0 7. 2 Feed rate, Sulphur, tons/ day 66. 7 40. 5 31.9 58.8 Feed rate, Spent Acid,

tons/day 101. 0 117. 2 46. 0 Total 100% HZSOl Produced, tons/day 200 200 200 300 Operating Pressure, lbs./

gal 44.1 44.1 44. l 44. l Converter Temp., F 750 750 750 750 Converter Diameter, I.

lyst, pounds/hr 900,000 180, 000 550,000 2, 130, 000 Air required, pounds/hr.

(10% excess) 39, 300 24, 300 31, 700 52, 400 Catalyst Cooler-Outlet Temperature, F 610 610 610 610 Excess heet to be removed by cooling catalySt, B. t. 11./hr 22, 900, 009 4, 600, 000 13, 900, 000 54, 200,000 Cooling `Duty on Reaction Products, 750 F.

to 60 F., B. t. u./hr 13, 100, 000 18, 700, 000 20, 000, 000 24, 500,000 Per cent HQSO; from Sulphur 100. O 60.7 49 90. 3 Per cent H2SO4 from Spent Acid 39. 3 51 9. 7

CATALYST Oxidation of sulphur to sulphur dioxide proceeds rapidly at all temperatures useful in the subsequent oxidation 0f sulphur dioxide to sulphur trioxide. However, the latter oxidation proceeds slowly in the abence of a catalyst. Also, while high temperatures increase the reaction rate, the equilibrium for this reaction is most favorable at relatively low temperatures and generally a temperature of 1000 F. is too high for commercial operation, resulting in too low a conversion. It is, therefore, a prerequisite that the catalyst be eifective to promote the oxidation of SO2 to S03 at a temperature at which the equilibrium is above for S03 and preferably is as close to as possible. The useful catalysts are generally provided by platinum or vanadium, for these are the only elements thus far adapted industrially and capable of effecting oxidation of SO2 rapidly at temperatures well below 900 F.

Platinum is usually disposed on a suitable carrier such as asbestos, diatomite, silica -gel or magnesium sulphate. Since platinum is poisoned easily it can only be utilized with sulphur of a suitable grade, While it is too expensive to be practical for an operation wherein acid sludges and like materials are used, owing to the danger of poisoning the catalyst with impurities in the sludges.

The vanadium catalysts include generally a, mixture of vanadium pentoxide, reactive silica and an alkali salt such as potassium sulphate, the mixture being disposed on a suitable carrier which may provide the required reactive silica. The manufacture of catalysts including vanadium is Well known and one may refer to Slama-Wolff Patent Re. 19,232 (Example 2 thereof) and to such patents as 1,657,753, 1,675,754, Re. 18,380, 1,675,330, 1,694,123 and 1,739,853. Various catn alyst carriers and catalyst modications are shown in Patents 1,463,206, 1,633,694, 1,695,740, 1,862,825 and 1,880,678.

Another catalyst mentioned in the literature is iron oxide, although oxides of other metals such as chromium, manganese, thorium, tin, titanium and tungsten, are mentioned. iron oxm ide is only eiective at a temperature of 1200" F. and above, where the equilibrium is quite unfavorable (according to Fairlie, Sulphuric Acid Manufacture, Reinhold, 193V, page 43, only about 62% conversion can -be attained at this tempel'u ature). While the use of iron oxide in the proce ess is not impractical, it Would be necessary to use it only for the rst conversion stage and to interpose one or more fixed catalyst beds between precipitator 53 and absorber 62 and to employ a catalyst (such as vanadium or platinum) promoting the reaction at a temperature lvvhereat more favorable equilibrium could be attained. In case one or more fixed catalyst beds are suitable coolers should be provided.

The catalyst employed docs not provide any limitation on the process. One can utilize any catalytic material which promotes the reaction at a temperature favorable to the equilibrium providing that (a) the material is not poisoned, as platinum may be, (1J) the material does not interfere with the process in some Way and (c) the material does not interfere with utilization of the sulphur trioxide or the acid.

ExrRANEoUs SULPHUR Dioxins If a gas containing a useful concentration oi sulphur dioxide is available from another source, e. g., the roasting of pyrites, this can be utilized to advantage, being injected directly into conn verter il, through line 30 or else mixed With the air stream employed to pick up the catalyst. To realize the full benet of the process, the sulphur dioxide furnished from an extraneous source should provide only a minor portion of that converted to sulphur trioxide and the major portion of the sulphur dioxide converted to sulphur trioxide should be generated in the system and in the presence of the catalyst.

I claim:

1. A process for producing S03 comprising injecting a material from the group consisting of sulphur, Waste sulphuric acid and mixtures thereof into a reaction zone, introducing solid particles of a vanadium oxidation catalyst and air into said zone to maintain a catalyst bed in fluidized suspension therein, the quantity and temperature of catalyst introduced being sufficient to maintain said zone at a temperature in the range of 600-1100 F, and conducive to exi-- dation of SO2 to S03, removing a stream of catalyst from said zone, cooling the removed catalyst stream, and returning at least a portion of the cooled catalyst stream with additional air.

2. A process for producing S03 comprising injecting a material from the group consisting of sulphur, Waste sulphuric acid and mixtures thereof into a reaction zone, introducing solid particles of a vanadium oxidation catalyst and air into said zone to maintain a catalyst bed in fluidized suspension therein, the quantity and temperature of cata-lyst introduced being suflicient to maintain said Zone at a temperature in the range of 600-1100 F. and conducive to oxidation of SO2 to S03.

3. A process for producing S03 comprising iniecting a material from the group consisting oi sulphur, Waste sulphuric acid and mixtures thereof into a reaction zone, introducing air and solid particles of an oxidation catalyst for sulphur and hydrocarbons into said Zone to maintain a catalyst bed in iiuidized suspension therein, the quantity and temperature of catalyst introduced being suflicient to maintain said zone at a temperature in the range of 6001100 F. and con duciv'e to oxidation of SO2 to S03, removing a stream of catalyst from said zone, cooling the removed catalyst stream, and returning at least a portion of the cooled catalyst stream with additional air.

4. A process for producing S03 comprising injecting a, material from the group consisting of sulphur, Waste sulphuric acid and mixtures thereof into a reaction zone, introducing air and solid particles of an oxidation catalyst for sulphur and hydrocarbons into said zone to maintain a catalyst bed in iuidized suspension therein, the quantity and temperature of catalyst introduced being suiiicient to maintain said zone at a temperature in the range of 600-1100 F. and conducive to oxidation of SO2 to S03.

ARNOLD BELCHETZ.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,355,105 Canon Oct. 5, 1920 1,473,879 Rudolph Nov. 13, 1923 2,337,684 Scheinman Dec. 23, 1943 2,373,008 Becker Apr. 3, 1945 

